Molecular structure features and fast identification of chemical properties of metal carboxylate complexes by FTIR and partial least square regression

Abstract

Several chemical properties of complexes should be considered to base their suitable applications in agronomic and environmental domains. However, the determination of these characteristics is generally expensive and time consuming. Therefore, the prediction of properties in complexes using a simple and expeditious technique, such as infrared spectroscopy, is highly desirable. This study aimed to investigate the molecular structure and chemical properties of metal complexes, such as stability, metal complexed fraction, and solubility. The complexes were submitted to experimental FTIR analysis and the spectroscopic data were regressed against the above-mentioned properties using partial least squares (PLS) to obtain a predictive model. Low molecular weight carboxylic acids (citric acid, malic acid, tartaric acid, and oxalic acid) were used as complexing agents for three metals (Mn, Zn, and Fe) in two stoichiometric ratios of reaction (1:1 and 1:2, metal: ligand molar ratio). Solubility and complexation ratio appeared to be dependent on the ligand and stoichiometry. Citrate and oxalate have formed the most stable complexes with the tested metals. Metal-oxalates tend to form 1:2 complexes at the pH and stoichiometries tested, whereas citrate, malate, and tartrate tend to form 1:1 complexes. All the complexes exhibited monodentate coordination of carboxylic groups with metals. Overall, the predictive models were built to estimate the solubility and the ratio of complexed metals. Furthermore, the coordination mode of ligands to metals could also be assigned from infrared band shifts of the carboxylate groups. This work revealed crucial chemical properties to base the use of complexes, and show that the fast and non-destructive FTIR technique can be used for predict these properties.